1. Field of the Invention:
This invention relates to a semiconductor laser device, more particularly to a GaAlAs/GaAs distributed feedback semiconductor laser device or distributed Bragg reflector semiconductor laser device.
2. Description of the prior art:
A GaAlAs/GaAs semiconductor laser device having a GaAs substrate and a heterojunction structure oscillates a laser beam at a wavelength of about 700 to 900 nm, and, therefore, is adequate as a light source for optical fiber communication, optical information processing, or optical measurement. Among semiconductor laser devices of such a kind, a distributed feedback (DFB) semiconductor laser device and distributed Bragg reflector (DBR) semiconductor laser device in which a periodical convexo-convex structure (diffraction grating) in formed in the vicinity of the active region have been intensively researched, because these laser devices oscillate at a single wavelength even when being modulated.
A process of manufacturing an example of such semiconductor laser devices will be described. FIG. 7 illustrates diagrammatically a prior art DFB semiconductor laser device. On a p-GaAs substrate 81, an n-GaAs current blocking layer 82 having a thickness of 0.8 .mu.m is formed by a liquid phase epitaxial growth method (LPE), metal organic chemical vapor deposition method (MOCVD), or the like. Then, using a photolithography technique and a chemical etching technique, a V-shaped groove 83 which reaches the p-GaAs substrate 81 is formed in the n-GaAs current blocking layer 82. The V-shaped groove 83 functions as a current path and forms an optical waveguide. Thereafter, a second LPE is conducted to form sequentially a p-Ga.sub.0.65 Al.sub.0.35 As cladding layer 84, a p-(or n-)GaAs active layer 85 having a thickness of 0.08 .mu.m, and an n-Ga.sub.0.93 Al.sub.0.07 As optical guiding layer 86 having a thickness of 0.3 .mu.m. The thickness of the p-cladding layer 84 on the groove 83 is 1.2 .mu.m.
Then, a diffraction grating G is formed on the n-Ga.sub.0.93 Al.sub.0.07 As optical guiding layer 86. An n-Ga.sub.0.65 Al.sub.0.35 As cladding layer 87 having a thickness of 1 .mu.m and an n-GaAs capping layer 88 having a thickness of 2 .mu.m are sequentially formed on the diffraction grating G by an LPE, thereby forming a laminated crystal structure of a double heterojunction type for laser oscillation.
The band gap of the optical guiding layer 86 is greater than that of the active layer 85 so that light propagates in both the active layer 85 and optical guiding layer 86, while the injected carriers are confined in the active layer 85. Such a structure is called an LOC (Large Optical Cavity) structure.
A prior art GaAlAs/GaAs DFB laser device having such a structure has a drawback which will be described. Since the GaAlAs optical guiding layer 86 on which the diffraction grating G is formed is easily oxidized, a stable oxide layer containing aluminum is formed on the surface of the optical guiding layer 86 in the step of the formation of the diffraction grating G is conducted. In the succeeding epitaxial growth (e.g., LPE or MBE), therefore, it is impossible to grow a uniform and highly crystallized GaAlAs layer. When an epitaxial growth is to be conducted on the diffraction grating G, hence, the AlAs mole fraction of the optical guiding layer 86 should be less than about 0.1 so that the layer 86 will not be easily oxidized. In a GaAlAs/GaAs laser device with an optical guiding layer having such a low AlAs mole fraction, the injected carriers cannot be confined in the active layer 85 unless the active layer 85 consists of GaAs. A laser device having a GaAs active layer oscillates a laser beam at a long oscillation wavelength (e.g., 870 nm). Therefore, it has been impossible to manufacture a GaAlAs/GaAs DFB or DBR laser device of shorter oscillation wavelength.